Add symlink method to installation docs

[5iver]

The documentation currently only instructs to copy files. We should add an option to clone the repo, use symlinks in the OH directories, and do a git pull to update the libraries and scripts. These are the links I use, in case someone gets to this before I do...

ln -s /opt/openhab-helper-libraries/Core/automation/lib/python/core /opt/openhab2/conf/automation/lib/python/core
ln -s /opt/openhab-helper-libraries/Core/automation/lib/javascript/core /opt/openhab2/conf/automation/lib/javascript/core
ln -s /opt/openhab-helper-libraries/Core/automation/lib/groovy/core /opt/openhab2/conf/automation/lib/groovy/core

ln -s /opt/openhab-helper-libraries/Core/automation/jsr223/python/core /opt/openhab2/conf/automation/jsr223/python/core
ln -s /opt/openhab-helper-libraries/Core/automation/jsr223/javascript/core /opt/openhab2/conf/automation/jsr223/javascript/core
ln -s /opt/openhab-helper-libraries/Core/automation/jsr223/groovy/core /opt/openhab2/conf/automation/jsr223/groovy/core

ln -s /opt/openhab-helper-libraries/Community/OpenWeatherMap/automation/jsr223/python/community/openweathermap /opt/openhab2/conf/automation/jsr223/python/community/openweathermap